Conventional disk drives employ a servo system that controls the radial position of an actuator arm relative to the surface of a rotating recording disk. The actuator arm supports a read/write head or transducer above a disk surface and ideally over the center of a selected track on the disk surface. For positioning purposes, the read/write head senses servo information embedded in the disk, which is then used to develop a position error signal. The error signal is then used to adjust the position of the read/write head in a direction to reduce the magnitude of the error for purpose of track following. The servo system is also utilized to move the read/write head from one track to another track.
At one time, disk drives were employed primarily within desktop computers, where the disk drives operated in a static environment within the computer on a desktop or table. The computer was in a stable position and there was little likelihood that disk drive would experience shock loading from impacts as a result of being dropped. Today, an increasing percentage of disk drives are being used in portable electronic devices, including laptop computers, notebook computers, palm-held devices, personal digital assistants, music players and other portable electronic devices. A primary problem associated with contemporary designs of such disk drives concerns shock-induced damage from the impact of a falling portable electronic device colliding with a surface. For example, when a device containing a small disk drive falls from a desk or a user's hand onto a hard surface, the shock pulse magnitude generated can be hundreds or thousands times the acceleration of gravity. Shock induced damage to the disk drive or its components is typically less a problem when the portable electronic device is turned off. When turned off, the actuator arm and head element are securely parked at a position off the surface of the disk or on a crash zone on the disk surface. In many cases, the actuator arm may also be latched to further inhibit movement away from the parked position. Therefore, if the portable electronic device is dropped, there is a substantially reduced likelihood that the disk surface and/or head will be damaged. Conversely, if the disk drive is in operation at the time of the fall, the actuator arm is unlatched and the head is likely positioned over the data portion of the disk surface. As a result, damage can easily occur to the disk surface and/or head element.
External shocks such as these yield at least two undesirable outcomes: physical damage of the disk and/or the head and track misregistration. During operation, a shock of sufficient magnitude will cause the head to impact the rotating disk, thereby damaging the magnetic media film, the disk substrate, and/or the head. Further, the shock event generates linear and radial accelerations that apply a moment to the actuator arm. This moment may exceed the ability of the servo system to maintain the read/write head within the allocated tracking error range required for acceptable data integrity, and the servo system may lose track of the actual position of the head element. This problem is exacerbated by increased track density which reduces the acceptable tracking error range. If a shock occurs during the data writing process, the disk drive is in jeopardy of miswriting the data off track, or worse, writing over previously written data on adjacent or nearby tracks.
Thus, it is often advantageous to ensure that the disk drive heads are in a parked position away from the rotating disks prior to impact or positioned over a designated crash zone. In the case of portable computers, this has been accomplished in the past by adding a micro-electro-mechanical-system (MEMS) accelerometer to the computer so that the free fall condition is sensed and the heads are parked prior to impact. For example, some MEMS accelerometers include an outer ring of material that is fixed to a stationary object, such as the motherboard of a computer. A suspended, movable mass is interconnected via a plurality of arms to an inside surface of the outer ring of material. As the MEMS accelerometer is accelerated, inertia causes the resting suspended mass to move relative to the outer ring thereby loading the plurality of arms that connect the mass to the ring. The arms are doped with a piezo-electric material that creates a voltage difference within the arms when loaded. The amount of voltage difference across each of the arms is measured to ultimately yield the magnitude of acceleration. When a disk drive is at rest, for example, sitting on a table, the acceleration measured by the accelerometer is 1 g (where g=force of gravity: 9.8 m/s2). The suspended mass of the MEMS accelerometer will be acted on by gravity and displaced downwardly from the outer ring causing a reading of 1 g acceleration. When the disk drive is dropped, the mass will move relative to the fixed ring, either in line therewith, causing a 0 g acceleration reading, or moving upwardly therefrom, causing a less than 1 g acceleration reading. Thus, when an acceleration indicates less than, or equal to, a predetermined threshold values for a predetermined amount of time, the disk drive is in a free fall condition. Once it is ascertained that the disk drive is indeed experiencing free fall, the voice coil motor that controls the position of the actuator arm is directed to place the actuator arm into a safe location, i.e., to park the actuator arm. When parked, the read/write head or transducer is located away from the rotating disks or over a crash zone so that should the disk drive impact a surface, the head does not strike the disk surface or is already in contact with the surface at a safe zone.
As an alternative, other MEMS accelerometers include a movable mass with a plurality of fingers emanating therefrom that interact with stationary fingers interconnected to a substrate. When at rest, a uniform gap exists between each pair of moveable and stationary fingers. When the mass of the accelerometer moves with respect to the stationary fingers, the gap between each set of fingers is either increased or decreased. The pairs of fingers function as capacitors, altering the space therebetween which changes the capacitance, which, in turn, is measured to identify the magnitude of the acceleration.
The prior art includes the use of accelerometers to detect free fall. U.S. Pat. No. 5,982,573 to Henze (“Henze”), which is incorporated by reference in its entirety herein, discloses a method of sensing acceleration using a MEMS accelerometer and moving the heads away from the disks before an impact occurs. The accelerometer employed is mounted in and secured to the housing of the disk drive. Thus, after a free fall event is detected, a signal is sent from the accelerometer to a processor to cause a signal to be sent to the voice coil motor to park the actuator arm. In other prior art devices, the accelerometer is positioned outside of the disk drive, such as on the motherboard of a computer. In these instances, the command to park the actuator arm must pass through the ATA interface, or similar interface, of the disk drive, and the disk drive must hold the current operation to respond to the command. In each instance, the interface, command, and response time and overhead involved slow or delay any action taken in response to the generated signal. This time lag can be directly correlated to lost reaction time and translates to a minimum drop distance for which corrective action cannot be taken. Conversely, only drops greater than this minimum distance may be detected in time to take corrective action. Unfortunately, even drops less than this minimum distance may produce considerable damage to a disk drive. Moreover, by placing the accelerometer outside of the disk drive, such as on the mother board of a computer, any malfunction of the computer can prevent the signal from the accelerometer from being processed and/or the appropriate corrective signal from reaching the voice coil motor.
In some instances, free falling of an object is accompanied by a rotation movement or tumble. The rotation generates a centrifugal acceleration, which results in the reading of the accelerator to be larger than zero during free fall. As a result, a detection system may not be able to reliably detect free fall with tumble using only the accelerometer as a free fall detection and protection mechanism.
Another known way to detect free fall/tumble event of a disk drive is to measure changes in velocity of the spinning disks. More specifically, it is known that an angular change of the axis of rotation of a spinning object will directly or indirectly alter the speed of that spinning object. In many instances, a portable electronic device experiences a tumbling action as a precursor to or as part of a free fall event. Thus, when a disk drive is experiencing a tumbling action, the rotational velocity of the spinning disks will necessarily change due to a load placed on the spindle from the change in the axis of rotation. One method of detecting free fall accelerations by measuring disk velocity is disclosed by U.S. Pat. No. 6,101,062 to Jenn et al. (“Jenn”), which is incorporated by reference in its entirety herein. Jenn discloses a method of monitoring spindle motor current in order to determine any change in the revolutions per minute (RPMs) of the spinning disks. As a change occurs in the plane in which a spinning disk operates, a load will be placed on the spindle bearings which, in turn, will slow the RPM of the spinning disk. Additional current will be needed to bring the RPMs back to the appropriate level. As a result, by utilizing an additional sensor, a tumble condition may be determined by monitoring the spindle motor current. However, there is a time lag between spindle speed change and motor current change. By monitoring motor current change to detect tumble, some reaction time will be lost for which corrective action cannot be taken.
In some instances, the disk drive may not change orientation as it falls, namely, where the change in angular momentum of the spinning disk is 0, i.e. a non-tumbling free fall. Therefore, no change of angular velocity of the disk drive would be readily apparent using this detection method and the existence of the tumble would not necessarily be detected or would not be detected in a timely manner to take corrective action. In addition, as noted, monitoring spindle motor current may require additional hardware and add to the cost of the disk drive.
Another drawback of the prior art devices and methods for detecting a fall is that they may be fooled to believe that the electronic device is free falling when it is not falling. More specifically, often vibrational loading of the system may be incorrectly identified as a free fall causing an unwanted parking of the head. For example, during travel on a train, airplane, bus or car, or during jogging or dancing, electronic devices are exposed to periodic vibrational accelerations. These vibrations may have an extended duration that may cause a detector to falsely conclude a free fall event is occurring and cause the heads to be parked. Similarly, jitter experienced by a spindle may create a false belief that a tumble event is occurring. Sources of spindle jitter include bearing load variations, electronic noise, windage, and magnetic coupling.
Thus, it is a long felt need in the field of disk drive protection to provide a method of more accurately detecting free fall so that the head can be parked prior to impact under all falling condition. There is also a need to more quickly determine if a disk drive is in free fall in order to reduce the height from which corrective action may be taken. In addition, a system is needed that allows for innocuous vibrations to be disregarded thereby preventing false indicators of a free fall event. The following disclosure describes an improved method of detecting disk drive free fall and tumbling that helps prevent misdiagnosis of these harmful events.